The present invention relates to sulfur-vulcanizable rubber compositions having a silica filler incorporated with optimization of the silica associated properties relating to wear, wet traction and rolling resistance. The silica associated properties are promoted by dispersing and coupling agents and, more particularly, the silica dispersion is improved through the use of a dispersing agent or aid comprising a quaternary ammonium compound and the timely addition thereof during the rubber mixing or blending process so as to not unnecessarily inhibit the coupling agent enhancements. The rubber compositions are particularly useful for tire tread applications in vehicles, for example passenger automobiles and trucks.
In recent rubber compounding developments, silica has been added to tire tread compositions as a filler to replace some or all of the carbon black filler. Although more costly than carbon black, the advantages of silica include improved wet traction with reduced fuel consumption. The disadvantages encountered in the use of silica include processing difficulties and reduced wear. The wear reduction is believed to be associated with the less uniform silica dispersion properties and decreased reinforcement characteristics of silica as compared with carbon black.
The difficulties in dispersing silica are believed to be associated with the polar nature of silica and non-polar nature of rubber. Regardless, the silica has been found to form localized clumps or aggregations in the rubber which detract from both tire wear and fuel consumption. As compared with carbon black, silica does not react as strongly with the rubber and thereby tends to give less strength to the rubber.
It is known to use coupling agents to enhance the rubber reinforcement characteristics of silica. A coupling agent is a bi-functional molecule that will react with the silica at one end thereof and cross-link with the rubber at the other end thereof. In this manner, the reinforcement and strength of the rubber are improved, e.g. the toughness, strength, modulus, tensile and abrasion resistance are particularly improved. The coupling agent is believed to cover the surface of the silica particle which then hinders the silica from aggregating with other particles. By interfering with the aggregation process, the dispersion is improved and therefore the wear and fuel consumption are improved.
The use of silica in relatively large proportions for improving various tire properties has been found to undesireably slow the cure rate of the rubber. This has led to the use of secondary accelerators such MBT, MBTS or DPG to maintain an adequate cure rate.
U.S. Pat. No. 4,602,052 discloses that small amounts of quaternary ammonium compounds may be used to speed up the cure rate, increase tensile and modulus, and decrease hysteresis in natural rubber containing compositions incorporating carbon black. The quaternary ammonium compounds are used as coupling agents for the carbon black filler. This patent references U.S. Pat. Nos. 3,686,113, 3,689,452 and 3,840,382 which are directed to aqueously wet never dried silica pigment precipitate or wet silica. The patent teachings further relate the preparation of elastomer-silica master batches, to silica coated with oleophylic quaternary ammonium material that is dried and dispersable in rubber, and to beads of silica combined with quaternary ammonium compounds and combined with other compounds such as processing oil and carbon black.
U.S. Pat. No. 3,627,723 teaches an alpha-olefin/non-conjugated diene copolymer with a clay or (carbon black filler using surfactants to improve the extrudability and vulcanizate properties of the copolymer. The surfactants include quaternary ammonium compounds, and silica is mentioned as a possible filler.
U.S. Pat. No. 3,998,778 discloses a method of making a powdered and flowable rubber composition including carbon black filler and a quaternary ammonium compound as an emulsifier. U.S. Pat. No. 4,032,501 also relates to emulsion preparation using a carbon black filler and a quaternary ammonium compound as an emulsifier.
U.S. Pat. Nos. 4,861,842 and 5,187,239 teach the use of quaternary ammonium compounds as secondary accelerators in rubber compositions.